1-(3-C-Ethynyl-β-D-ribopentofuranosyl)cytosine (ECyd, represented by the following formula) is an antimetabolite having a structure in which the 3′-β-position of the ribose of cytidine is substituted by an ethynyl group.

ECyd is a cytidine derivative which was first synthesized in Japan. Differing from a pyrimidine derivative (5-FU) or a deoxycytidine derivative (gemcitabine), which are antitumor agents generally employed in the clinical settings, ECyd weakly acts on DNA and mainly inhibits RNA synthesis. Specifically, in a proposed mechanism, ECyd is phosphorylated by intracellular uridine/cytidine kinase, to thereby form a triphosphate (ECTP), which inhibits RNA polymerases I, II, and III, leading relevant cells to death (Patent Document 1 and Non-Patent Document 1).
Many antitumor agents which are generally employed in the clinical settings and which work based on DNA synthesis inhibition as a main action exhibit the inhibitory effect in an S-phase. Tumor cells employed in animal tests generally exhibit relatively fast proliferation. However, studies have revealed that, in the clinical settings, tumor cells proliferate at a slow rate, and a small number of the cells are in the S-phase. Since, differing from DNA synthesis inhibiting agents, the antitumor effect of ECyd based on RNA synthesis inhibitory action is not affected by the cell cycle of tumor cells, ECyd is thought to serve as a clinically useful antitumor agent, which differs from DNA synthesis inhibiting agents generally employed in the clinical settings.
Another attempt has been made to further potentiate the antitumor effect of ECyd, in which an antitumor agent such as cisplatin, having a different action mechanism, is employed in combination (Non-Patent Document 2). However, hitherto, a satisfactory potentiating effect has not been attained. Therefore, there is keen demand for a new combined therapy with ECyd, which therapy exhibits a more potent antitumor effect and gives less adverse effects.